The prior art is described with help of following figures.
FIG. 1 shows sectional view of H-H type heat exchangers described below.
FIG. 2 shows sectional view of the H-L type heat exchangers described below.
FIG. 3a shows the part sectional view of the gasketed joint in the channel header.
FIG. 3b shows the part sectional view of the gasketed joint in the channel header in its alternative arrangement.
Threaded channel closure type heat exchangers are generally classified based on the operating pressure on the shell side and tube side. The heat exchangers having high pressure on both the shell side as well as tube side are classified as H-H type heat exchangers, while the heat exchangers having lower pressure on shell side and high pressure on tube or channel side are classified as H-L type heat exchangers.
Due to these conditions, in case of H-H type heat exchangers the tubesheets themselves are subjected to lower differential pressure. Consequently, H-H type would typically have internal tubesheet with an apparatus for sealing of tubesheet against shoulder of the channel.
In H-L type, there being usually higher pressure on channel side and lower pressure on shell side, the tubesheets get typically exposed to high differential pressure. The tubesheets and channel covers in this case are typically of integral construction, either single piece or welded together.
Tubesheets are provided with plurality of holes in which tubes (5) are fixed. The channel is provided with nozzles (6) for the tube side fluid to enter and exit the heat exchanger. The heat exchangers are preferably provided with two or more tube passes. This is achieved by provision of partitions and covers inside the channel in a known way.
Both H-H and H-L type heat exchangers have channel headers (1) provided with closure consisting of a channel cover (3) and threaded lock ring (2) to retain the cover (3). The threaded lock ring (2) is screwed in the threads provided in the channel header body.
A gasketed joint is provided to seal the closure. A gasket (7) is located in the groove (11), in the shoulder of the channel as shown in FIG. 3a; or in an alternative arrangement, in the shoulder (12) formed in the channel as shown in the FIG. 3b. The gasket is compressed by peripheral portion i.e. tongue (101) of the diaphragm (8) which enters the groove (11) so as to compress the gasket. The diaphragm (8) is backed by a compression ring (9) at the periphery and the channel cover (3) in the central portion. The channel cover (3) is held in position by the threaded lock ring (2). The push bolts/rods (10) fitted in the threaded holes at the periphery of the threaded lock ring (2) pressurize the compression ring (9) when tightened. The compression ring in turn presses the tongue of the diaphragm to seal the gasket by pressurizing it. The end thrust due to the internal pressure on diaphragm is transmitted to the outer compression ring (9) and threaded lock ring (2) and resisted by it. The push bolts/rods 10) provide incremental loading of the gasket through diaphragm for keeping the joint leak-proof.
It can be seen from the above arrangement that, for obtaining the leak-proof joint the tongue (101) of the diaphragm has to enter the groove (11). Together with the heat exchanger, the diaphragm (8) has to undergo multiple number of pressure/temperature cycles over a period of time and makes it prone to distortion and deformation. This can cause the diaphragm to shrink in outer diameter thus pulling the inner edge (103) of the tongue (101) inwards, thus riding over the inner edge (104) of the groove (11) provided in shoulder of the channel, during retightening of the push bolts/rods (10). The metallic contact thus developed between the tongue (101) of the diaphragm (8) and inner edge (104) of the groove can obstruct transmission of load generated by tightening of the push bolts (10), thus only partly loading the gasket. Due to this the gasket is only superficially compressed which may cause continuous minor leaks, which not only can remain unnoticed but also can dangerously build up pressure beyond gasketed joint ahead of threaded portion of the channel header. This could lead to unsafe condition for the equipment with potential risk of disastrous accidents.
Considering the alternative arrangement as depicted in FIG. 3b wherein a shoulder (12) is provided in place of the groove (11), it can be readily seen that, this arrangement makes the gasket (7) unconfined at its inner diameter. This can lead to uncontrolled compression of the gasket making the joint unreliable and hence unsafe.